1. Field of the Invention
The present invention relates to two-stage proportional directional control spool valves.
2. Description of the Prior Art
Solenoid-operated proportional control valves are known having a spool acted upon by a solenoid at each end and provided with centring springs. Should the electrical supply to the solenoids fail, the spool is brought to this central or null position by the centring springs, in which position the service ports of the valve are sealed so that the load is hydraulically locked in position. A single-stage valve is thus fail-safe in the event of electrical failure.
In another known arrangement, the pilot spool is acted upon by a single solenoid at one end with a return spring acting on the opposite end of the spool. With this arrangement, in the event of electrical failure to the solenoid, the return spring moves the spool to an offset, fail-safe position and thus, as with the two-solenoid arrangement, a positive fail-safe position of the pilot spool is achieved. The disadvantage of this arrangement is that as the pilot moves to the offset, fail-safe position, it may pass through the normal operating range of the pilot spool and thus transmit a short pulse of supply pressure fluid to one of the service ports which in turn causes the main valve to make a small movement away from null and thus move a load controlled thereby. This can give rise to difficulties in some applications.
The use of two solenoids thus offers an advantage over the use of a single solenoid because the pilot spool is centred as opposed to being offset, when there is an electrical failure. However, the use of two solenoids alone would not overcome the problem because the spool is ideally constructed so as to have as close as possible zero lap during normal operation and this does not give rise to a rapid or reliable fail-safe condition. Zero lap is required so as to be able to effect quick reversal of the main stage when required. Both underlap end overlap would introduce some delay in this regard which, however small, is undesirable in the context of precise control of the main stage. However, upon failure of the solenoids, the main stage needs to be centred quickly by equalising the pressure in the end chambers of the main spool and to achieve this, the pilot spool is ideally underlapped, whereby there is a conflict of requirements between normal operation and fail-safe operation.
In U.S. Pat. No. 4,617,967 there is disclosed a two-stage proportional control valve in which the pilot spool is split into two parts with a spring arranged to act between the two parts so as to urge them apart although in normal operation of the valve, the two parts of the pilot spool are maintained in contact with each other due to the forces exerted by the solenoids. When the solenoids are de-energised, intentionally or as a result of electrical failure, the spring moves the two parts of the spool away from each other, thereby changing the lap conditions and giving a well-defined fail-safe condition. However, the control valve disclosed in U.S. Pat. No. 4,617,967 has an arrangement of ports in the pilot valve to accommodate the split-spool arrangement, the port arrangements being in fact the reverse of the normal arrangement, whereby the usual single supply port is made the tank port and the usual two tank ports made the supply ports, these being linked by a gallery or loop. The tank loop in a valve normally has a low pressure rating so that if subjected to supply pressure, can give rise to valve body distortion unless, of course, the valve body is strengthened with the attendant disadvantage of increased manufacturing costs. If the normal, central pressure supply port is used with a split-spool arrangement, as disclosed in U.S. Pat. No. 4,617,967, a change-over block would have to be employed, again with increased manufacturing costs.